1. Field of the Invention
The present invention relates to a sealing system provided with a dust seal.
2. Description of the Related Art
For example, in oil hydraulic cylinders for construction machinery, a sealing system comprising a plurality of seals is used so as to seal an annular gap between a cylinder (housing) and a piston (shaft). Such a sealing system is provided with a main seal for preventing the leakage of fluid to be sealed, such as oil, and a dust seal for preventing dust from invading into a hermetic seal area. In general, only one dust seal made of urethane rubber is arranged at an atmospheric air side.
However, in cases where such a sealing system is used under a very severe environment, two dust seals are used while being arranged side by side. Here, note that as an example of a sealing system in the case of being used under a very severe environment, there is a case in which such a sealing system is used in an iron ore mining site, and dust includes, in addition to particulates, frozen substances, substances adhered to the surface of a piston and mud, etc.
Such a sealing system according to a conventional example will be explained with reference to FIG. 8 and FIG. 9. FIG. 8 is a schematic cross sectional view showing the state of the use of the sealing system according to the conventional example. FIG. 9 is a plan view schematically showing a second dust seal in the sealing system according to the conventional example.
The illustrated sealing system according to the conventional example is used for the purpose of sealing an annular gap between an inner peripheral surface of a shaft hole formed in a housing 500, and an outer peripheral surface of a shaft 300 inserted through the shaft hole. In addition, the sealing system according to this conventional example is provided with a main seal 600 for preventing fluid to be sealed from leaking to an atmospheric air side (A), and a dust seal 700 for preventing dust from invading to the side (O) of the fluid to be sealed. In this conventional example, the dust seal 700 is composed of a first dust seal 710 that is formed at the side of the main seal 600, and a second dust seal 720 that is formed at a location nearer to the atmospheric air side than this first dust seal 710.
Here, the housing 500 is composed of a housing main body 510, and a member 520 to be fastened (hereinafter also referred to as a to-be-fastened member) which is constructed so as to be freely fastened and unfastened with respect to this housing main body 510 by means of a bolt or the like.
An annular groove 511 is formed on the inner peripheral surface of the shaft hole in the housing main body 510. The main seal 600 is fitted in this annular groove 511. Here, note that in this conventional example, the main seal 600 is composed of a U-shaped packing 610 of a U shape in cross section, and a back-up ring 620 for protecting an inner peripheral end edge of the U-shaped packing 610 at the atmospheric air side (A).
In addition, an annular concave portion 512 forming a step is formed in an open end portion of the shaft hole in the housing main body 510 at the atmospheric air side (A). The first dust seal 710 is fitted in this annular concave portion 512. The first dust seal 710 is composed of a metal ring 711, and a seal body 712 that is integrally formed with this metal ring 711. This first dust seal 710 is fixed in an axially positioned state by fastening the to-be-fastened member 520.
Moreover, an annular concave portion 521 forming a step is formed in an open end portion of a shaft hole in the to-be-fastened member 520 at the atmospheric air side (A). The second dust seal 720 is fitted in this annular concave portion 521. Similar to the first dust seal 710, the second dust seal 720 is composed of a metal ring 721, and a seal body 722 that is integrally formed with this metal ring 721. This second dust seal 720 is fixed in an axially positioned state by fitting a snap ring 530 into an annular groove formed on an inner peripheral surface of the shaft hole in the to-be-fastened member 520.
In this manner, in the sealing system according to this conventional example, the two dust seals are used while being arranged side by side. Accordingly, it is possible to prevent dust from invading to the side (O) of the fluid to be sealed in a more reliable manner in comparison with the case in which only one dust seal is used.
Here, with respect to the first dust seal 710, there is no part cut in a circumferential direction thereof, similar to common dust seals. As a result, at the time of assembly, an operation of fitting the first dust seal 710 onto the shaft 300 from an end side thereof is needed, or an operation of inserting the shaft 300 into the first dust seal 710 is needed.
In contrast to this, with respect to the second dust seal 720, it is exposed to atmospheric air, so in cases where it is used under a very severe environment, the progress of deterioration thereof is rapid. For example, muddy water or the like would adhere and stick to the seal body 722, or the muddy water or the like adhered to the seal body 722 would be frozen, and when the shaft 300 is caused to slide relative to the seal body 722 in such a state, wear on a seal lip of the seal body 722 would be facilitated.
In this manner, the second dust seal 720 is apt to deteriorate, and hence, is constructed so that it can be replaced with a new one in an easy manner. More specifically, the second dust seal 720 is of a two-piece divided structure. That is, it is composed of a circular body, being circular as seen from an upper surface side, which is cut or divided into two semicircular shaped members, as shown in FIG. 9. With such a structure, each divided member can be detached and attached to the shaft 300 from an outer peripheral side thereof, as a result of which a replacement of the second dust seal 720 can be carried out in a simple manner only by unfastening the to-be-fastened member 520 from the housing main body, then removing the second dust seal 720, which has been deteriorated, attaching a new one, and finally fastening the to-be-fastened member 520 again.
However, the second dust seal 720 is prepared as follows. That is, from the view point of fabrication, a circular body, which is circular as seen from an upper surface side, is first prepared, and is then divided by cutting into two pieces. Here, the second dust seal 720 is provided with the metal ring 721, so in order to cut the metal ring 721, such a cutting has to be made by means of a cutting blade or edge having a certain amount of thickness. Here, note that in general, the cutting is done by the use of a grinder, etc.
As a result, a part of the metal ring 721 and a part of the seal body 722 are chipped off by cutting (C1 and C2 in FIG. 9 indicate the parts to be chipped off). Accordingly, even if the two divided members are joined or combined with each other, a perfect circle cannot be formed, and hence, in a state where these members are attached to the housing 500, it cannot be avoided that a gap occurs between the cut ends of the members. Specifically, the gap of about 2.5 mm at the maximum occurs. For that reason, the invasion of dust from this gap cannot be prevented.
As a measure to cope with this, the applicant of this application has already filed an application (Japanese patent application 2007-63531) with respect to a technique which adopts, as a second dust seal, one which is made of only urethane rubber, and which is cut only at one location in a circumferential direction by means of a thin cutting edge which is so thin as to make a chipping allowance unnecessary.
However, in the case of this technique, the second dust seal has a cut portion, so it is particularly difficult to prevent the invasion of minute or fine dust to a sufficient extent. Once dust enters from the second dust seal, wear of a seal lip thereof will be facilitated, and the invasion of dust will be promoted.
The dust having entered from the second dust seal is prevented from further coming into a main seal side, by means of a first dust seal. However, the dust accumulates in a sealed area between the first dust seal and the second dust seal. The dust thus accumulated in this sealed area, being in a state sealed by the first dust seal and the second dust seal, cannot escape from this sealed area. Accordingly, when the amount of accumulation of the dust exceeds a predetermined amount, a load (pressure due to the accumulated dust) is applied to the first dust seal and the second dust seal. Thus, there is a fear that the first dust seal and the second dust seal may be damaged by this load.
The accumulation of dust is more apt to occur in the case of minute dust (i.e., dust having a particle diameter of 10 μm or less) such as particulates than in the case of frozen substances, mud and the like. In addition, in cases where the second dust seal is composed of a relatively hard material such as urethane rubber, such minute dust can also enter not only from the cut portion but also from the tip side of the seal lip.
In this manner, the dust which has entered from the second dust seal and has accumulated becomes a cause of damaging the first dust seal and the second dust seal, thus resulting in a cause of shortening the lives of these dust seals.
Here, note that as related techniques, there are those which are disclosed in a first through a fourth patent document, as listed below.